1. Field of the Invention
The present invention relates to a method for controlling an idle stop mode in a hybrid electric vehicle, and more particularly, to a method for controlling an idle stop mode in a hybrid electric vehicle in which an idle stop mode is entered when a deceleration of a hybrid electric vehicle is more than a medium deceleration, whereby a fuel consumption ratio is improved.
2. Description of the Related Art
A typical hybrid electric vehicle, as shown in FIG. 1, comprises an inverter 10, a DC/DC converter 20, a high voltage battery 30, a hybrid control unit (HCU) 40, a motor control unit (MCU) 50, a battery management system (BMS) 60, an engine control unit (ECU) 70, a transmission control unit (TCU) 80, a clutch and a continuously variable transmission (CVT) 90, an engine 100, and a motor 200. The engine 100 and the motor 200 are serially connected to each other and serve as a power source for driving a vehicle. The clutch and CVT 90 serve to transfer a power. The inverter 10, the DC/DC converter 20, and the high voltage battery 30 serve to drive the engine 100 and the motor 200. The hybrid control unit (HCU) 40, the motor control unit (MCU) 50, the battery management system (BMS) 60, the engine control unit (ECU) 70, and the transmission control unit 80 serve as means for controlling the above-described components and are connected to communicate with each other through controller area network (CAN) communications.
Functions of the components of the hybrid electric vehicle are described below.
The HCU 40 is an upper-level controller which controls an overall operation of a hybrid electric vehicle. The HCU 40 communicates with the MCU 50, which is a sort of a low-level controller, to control torque, speed and power-generation torque of the motor and communicates with the ECU 70, which controls the engine for generating a power for voltage generation as a power source, to perform an engine starting-related relay control operation and a fault diagnosis operation.
The HCU 40 also communicates with the BMS 60, which manages an overall state of a battery by detecting a temperature, a voltage, an electrical current, a state of charge (SOC) of a battery which is a main power source, to control torque and speed of the motor according to the SOC. The HCU 40 also communicates with the TCU 80, which determines and controls a transmission gear ratio according to a vehicle speed and a demand of a driver, to perform a control operation for maintaining a vehicle speed required by a driver.
The HCU 40 monitors information (accelerator or brake) requested by a driver and current states of the MCU, BMS, ECU, and TCU to control an output voltage of the DC/DC converter so that energy can be efficiently distributed according to a vehicle state. Here, the DC/DC converter 20 serves to have a power to be supplied for a vehicle electrical equipment load and a 12V battery to be efficiently charged.
The high voltage battery 30 is an energy source for driving the motor and the DC/DC converter 20 of the hybrid electric vehicle. The BMS 60 which is a controller of the high voltage battery 30 monitors a voltage, an electrical current and a temperature of the high voltage battery 30 to control the SOC (%) of the high voltage battery 30.
The inverter 10 receives energy from the high voltage battery 30 to supply a three-phase alternating current necessary for driving the motor 200, and the MCU 50 controls the motor 200 under control of the HCU 40.
In connection with control of the DC/DC converter 20, the ECU 70 and the TCU 80 receive an accelerator pedal effort and a brake signal of a driver and provide related information to the HCU 40, which is an upper-level controller, to determine vehicle charging energy.
As an accelerating pedal, i.e., accelerator, a hybrid electric vehicle usually utilizes an electronic throttle control (ETC) type, and when a driver pushes an accelerating pedal, it is converted into a driver requesting torque form, so that torque suitable for a vehicle speed is determined.
That is, the driver requesting torque is set to a mapping value of a vehicle speed for a detecting value of an accelerating pedal, and operating points of the motor, the generator and the engine are determined according to the determined driver requesting torque.
One of main purposes of such a hybrid electric vehicle is to realize a high efficiency vehicle with a high fuel consumption ratio and an eco-friendly vehicle with high emission performance.
In order to achieve the above purpose, a hybrid electric vehicle employs an idle stop mode. Here, the idle stop mode is a mode to stop idling of the engine when a vehicle stops. Due to the idle stop mode, unnecessary idling of the engine is prevented, thereby improving a fuel consumption ratio and emission performance.
When the idle stop mode is triggered to stop the operation of engine, a power of the engine and the motor is transmitted to a vehicle through a transmission, i.e., CVT. Therefore, in order to stably enter the idle stop mode, the clutch, the engine and the motor should be organically controlled.
In order to enter the idle stop mode, the HCU 40 transmits the idle stop mode triggering signal to the ECU 70, the TCU 80 and a full auto temperature control (FATC) (not shown), so that the TCU 80 disengages the clutch to prevent a power of the engine and the motor from being transmitted to a vehicle, and the ECU 70 turns off an engine to prevent a power of the engine from being transmitted.
At this time, the HCU transmits a signal to the MCU 50 to have kill torque to be generated in the motor, so that remaining torque of the engine and the motor is removed, whereby the idle stop mode is completely entered.
However, a conventional hybrid electric vehicle has a problem in that the idle stop mode is not smoothly or stably entered when a deceleration is large (e.g., less than −2 m/sec2), leading to high fuel consumption ratio.
Such a problem is caused by the following reason.
If a deceleration of a hybrid electric vehicle is large (e.g., less than −2 m/sec2), a gear ratio of the CVT does not reach a target minimum gear ratio with a big difference as shown in a graph of FIG. 4, so that the TCU performs a control operation for preventing the idle stop mode from being entered and for having a gear ratio to become a target gear ratio in a state that revolution per minute (RPM) of the engine is secured.
The reason why a gear ratio does not reach a target gear ratio is because if a deceleration is large, large braking torque is required, and so it is difficult to form an oil pressure for changing a gear. For example, it gets delayed to form an oil pressure for suddenly changing a gear ratio from a fourth gear to a first gear. If an oil pressure for changing a gear ratio is not formed, regenerative braking begins during deceleration, so that a gear ratio does not reach a target gear ratio.
If a gear ratio of the CVT does not reach a target gear ratio, the TCU transmits an idle stop mode preventing signal for prohibiting the idle stop mode to the ECU to perform a control operation for obtaining a gear ratio for restart.
Here, if restart is performed in a state that a gear ratio of the CVT does not reach a target gear ratio, a problem such as engine stalling and engine hesitation occurs. For example, it is similar to a phenomenon such as engine stalling and engine hesitation which occur when a manual transmission vehicle starts in a third gear or a fourth gear at a speed of less than 10 kph.
That is, if an actual gear ratio (2.1) of an idle stop mode entering vehicle speed time point does not reach a target minimum gear ratio, the TCU transmits an idle stop mode preventing signal to the ECU to perform a control operation for obtaining a gear ratio for restart.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known to a person skilled in the art.